Create sub assets throws nullpointer

We are using the workflow to 'Dam Update Asset' to create renditions for all the new assets that are added in AEM.

And since yesterday we see the following error appearing in the log files:

18.07.2017 07:24:15.270 *ERROR* [JobHandler: /etc/workflow/instances/server1/2017-07-18/model_6797382541536:/content/dam/test/homepage/2017/06-19/homepage-logo.png/jcr:content/renditions/original] com.day.cq.workflow.compatibility.CQWorkflowProcessRunner Process execution resulted in an error: null
java.lang.NullPointerException: null
        at com.day.cq.dam.core.process.CreateSubAssetsProcess.execute(CreateSubAssetsProcess.java:50)
        at com.day.cq.workflow.compatibility.CQWorkflowProcessRunner.execute(CQWorkflowProcessRunner.java:93)
        at com.adobe.granite.workflow.core.job.HandlerBase.executeProcess(HandlerBase.java:184)
        at com.adobe.granite.workflow.core.job.JobHandler.process(JobHandler.java:232)
        at org.apache.sling.event.impl.jobs.JobConsumerManager$JobConsumerWrapper.process(JobConsumerManager.java:512)
        at org.apache.sling.event.impl.jobs.queues.JobRunner.run(JobRunner.java:205)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        at java.lang.Thread.run(Thread.java:748)
18.07.https://lh3.googleusercontent.com/-U2shZQNg6b8/WW20fbyonsI/AAAAAAAAWJQ/B4fjzGVfosgJzwvQ7QzBQSqIJDSU3f8FwCL0BGAYYCw/h1536/1833890570304360292%253Faccount_id%253D12017 07:24:15.270 *ERROR* [JobHandler: /etc/workflow/instances/server1/2017-07-18/model_6797382541536:/content/dam/test/homepage/2017/06-19/homepage-logo.png/jcr:content/renditions/original] com.adobe.granite.workflow.core.job.JobHandler Process execution resulted in an error
com.adobe.granite.workflow.WorkflowException: Process execution resulted in an error
        at com.adobe.granite.workflow.core.job.HandlerBase.executeProcess(HandlerBase.java:194)
        at com.adobe.granite.workflow.core.job.JobHandler.process(JobHandler.java:232)
        at org.apache.sling.event.impl.jobs.JobConsumerManager$JobConsumerWrapper.process(JobConsumerManager.java:512)
        at org.apache.sling.event.impl.jobs.queues.JobRunner.run(JobRunner.java:205)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
        at java.lang.Thread.run(Thread.java:748)
Caused by: com.adobe.granite.workflow.WorkflowException: Failed to execute process
        at com.day.cq.workflow.compatibility.CQWorkflowProcessRunner.execute(CQWorkflowProcessRunner.java:108)
        at com.adobe.granite.workflow.core.job.HandlerBase.executeProcess(HandlerBase.java:184)
        ... 6 common frames omitted
Caused by: java.lang.NullPointerException: null
        at com.day.cq.dam.core.process.CreateSubAssetsProcess.execute(CreateSubAssetsProcess.java:50)
        at com.day.cq.workflow.compatibility.CQWorkflowProcessRunner.execute(CQWorkflowProcessRunner.java:93)
        ... 7 common frames omitted

Can anyone help with finding out what is causing the nullpointer exception on line 50 ?

Regards

Steven